Valve guides are press fit into smaller diameter cylindrical mating bores contained in internal combustion engine heads to obtain a stable fit between the two components. A stable fit is desirable to prevent subsequent loosening during operation of the engine and to facilitate heat transfer between the components. Valve guide blanks are hollowed to obtain a center hole that surrounds and contacts the elongated cylindrical valve stem during operation of the engine. The valve stem moves axially during operation of the engine, and thus wear is created. While it is desirable in general to decrease wear between the valve guide and valve stem, it is desired to have the majority of wear occur on the surface of the interior center hole of the valve guide rather than on the valve stem. Excessive wear on the valve stem can lead to breakage of the stem and consequent catastrophic engine failure.
Blanks for subsequent fabrication into valve guides for use in internal combustion engines may comprise ferrous or non-ferrous metals and have been formed by powder metallurgy processes or from cast metals. The present invention relates to an improved process for making cast and heat-treated alloyed gray cast iron valve guide blanks having a hardness that is compatible with the hardness of the valve stem. Such compatibility minimizes wear to occur on the surface of the interior center hole of the valve guide rather than on the surface of the valve stem. This hardness compatibility serves to reduce the incidence of unacceptable excess exhaust emissions and possible engine failure. In addition, a microstructure of the alloyed cast iron valve guide comprising at least about 3% intercellular carbide in a tempered martensite matrix reduces wear of the internal surface of the valve guide.
Gray cast irons are well known in the art and may be used in the as-cast or heat-treated conditions. United States patents directed to such gray cast irons include U.S. Pat. Nos. 1,973,263; 3,384,515; 3,370,941; 3,472,651; 4,032,334; and 4,124,413.
Prior processes for making gray cast iron valve guides include two general approaches. A first approach involves the use of unalloyed gray cast iron that is subjected to heat treatment to obtain desired mechanical properties, such as increased tensile strength and impact resistance. Such heat treatments comprise austenitizing followed by quenching and tempering and are more fully described at pages 207-210 of Volume 2 of the 8th Edition of Metals Handbook entitled “Heat Treating, Cleaning and Finishing” (1964) and pages 363-365 of Volume 1 of the 8th Edition of Metals Handbook entitled “Properties and Selection of Metals” (1961). A second approach involves the use of alloying to modify such as-cast mechanical properties as strength and hardness. The effect of certain alloying elements is discussed in greater detail at aforesaid pages 363-365.
The present invention is specifically directed to the manufacture of alloyed gray cast iron valve guides with use of a process that includes heat-treating by austenitizing followed by quenching and tempering. Pages 209-210 of aforesaid pages 207-210 include Examples 1 and 2, which specifically deal with the heat treatment of unalloyed gray cast iron valve guides. The heat treatments disclosed involved austenitizing at 1600° to 1625° F., oil quenching, and then tempering at 900° to 925° F. to obtain a Rockwell C hardness of approximately 30 to 34.
In contrast with the above-discussed prior art activity, the present invention provides an improved manufacturing process, and resultant valve guide product, that is characterized by a heat treatment designed to achieve a tempered martensite microstructure that enhances machineability during the finish machining operation used to form the valve guide and results in a microstructure that reduces wear on the interior surface of the valve guide during operation.